This application is based upon and claims the benefit of priority of Japanese Patent Application No. H. 11-243389 filed on Aug. 30, 1999, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a tire air pressure warning device for indirectly detecting tire air pressure of a vehicle or a change of the same and warning a reduction of the tire air pressure without using pressure sensors provided for this purpose.
2. Description of the Prior Art
There has been proposed a tire air pressure detecting device in which the tire air pressure is indirectly detected as a substituting characteristic for the tire pressure, based on signals generated by wheel speed sensors. For example, there is known a tire air pressure warning system in which, in view of a fact that a resonance frequency (spring constant) of the tire may be changed according to variation of the tire air pressure, a resonance frequency (spring constant) fk estimated by data processing based on wheel speed signals is compared with a predetermined threshold value fo for determining tire air pressure, as disclosed in JP-A-5-133831.
In consideration of various forms that the tire air pressure for vehicle becomes abnormal, there is a case that the tire air pressure is slowly reduced by natural leakage and a case that the tire air pressure is reduced when the tire is punctured by a nail. In these two cases of abnormal tire air pressure, it is not always preferable to detect such abnormal tire air pressure during the vehicle running.
For example, in the case of natural leakage of the tire air pressure, it is not necessary to detect the tire air pressure during the vehicle running since the tire pressure is reduced gradually over several months, but it is preferable to detect the abnormal tire air pressure before starting the vehicle.
Further, in a radial type tire for a modern passenger car, the tire pressure is unlikely to be reduced instantaneously even if the tire is punctured. Therefore, it is more important to warn a driver without fail when the tire pressure becomes remarkably low after the vehicle, which has the flat tire, has been parked overnight than to warn the driver within the vehicle while driving when the tire is damaged.
It is thus an object of the present invention to provide a tire air pressure warning device capable of warning reduction of the tire air pressure by data processing in analyzing the wheel speed signals when the vehicle substantially stops.
To achieve the above object, the tire air pressure warning device comprises detecting means for detecting vibration of the tire axle caused by a pulsating force given to the tire from outside impact means within a period covering from a time when the vehicle stops to a time when the vehicle moves at a predetermined low speed, calculating means for calculating natural frequency or resonance frequency of the detected vibration, determining means for comparing the natural frequency or the resonance frequency with a preset value and determining a state of tire air pressure of the tire, and warning means for informing a driver the state of the tire air pressure. With the device mentioned above, the outside impact means generates a force causing a rotation variation of the tire axle. As the vibration based on the rotation variation of the tire axle reflects a change of the tire air pressure, the tire air pressure may be indirectly detected even when the vehicle substantially stops.
Further, it is preferred that the force causing the rotation variation of the tire axle is generated by intentionally changing a braking force to be applied to the tire axle in a pulsating way at a time when actual brake control for the vehicle such as anti-skid control and other brake control is not executed, for example, when the vehicle substantially stops.
Furthermore, it is preferable from a cost standpoint to execute data processes of signals for determining abnormal tire air pressure and to control the hydraulic brake circuit in use of ECU for the anti-skid control. In this case, a surplus burden can not be given to ECU for the anti-skid control since the tire air pressure warning device is operative only when the vehicle substantially stops and the anti-skid control is not executed.